(A) IL-6 stimulation of murine M1 cells augments the formation of a STAT3, Rac1·GTP and MgcRacGAP ternary complex. Rac1 and MgcRacGAP are also required for nuclear import of pSTAT3, with MgcRacGAP translocating with pSTAT3. MgcRacGAP is also required for STAT3 driven transcription of NF-kB. (B) Similarly, IL-3 stimulation of Ba/F3 cells augments the formation of a STAT5A, Rac1·GTP and MgcRacGAP ternary complex. Both Rac1 and MgcRacGAP are also required for the nuclear translocation of pSTAT5A, with MgcRacGAP moving simultaneously with pSTAT5A. MgcRacGAP dependent nuclear import of STAT5 is shown to be reliant upon an interaction between the MgcRacGAP NLS and importin-α, a relationship not currently tested for STAT3. MgcRacGAP is required for STAT5A driven transcription of Bcl-xL.